1. Technical Field
The present invention relates to a piston ring. The present invention further relates to a method for manufacturing the piston rings according to the invention.
2. Related Art
Piston rings in a combustion engine seal the gap that exists between the piston head and the cylinder wall off from the combustion chamber. As the piston moves up and down, the outer peripheral surface of the piston ring slides along the cylinder wall in permanently spring-biased contact therewith, while the piston ring itself oscillates as it travels in its piston ring groove due to the tilting movements of the piston, and this oscillation causes the flanks of the ring come into contact alternatingly with the upper and lower flanks of the piston ring groove. As the two elements slide over one another, each is subject to a certain amount of wear depending on the nature of the material, and in the event of dry running this can lead to seizing, scoring and ultimately cause irreparable damage to the engine. In order to improve the sliding and wearing behaviour of piston rings with respect to the cylinder wall, the peripheral surfaces of the piston rings have been coated with various materials.
The parts of internal combustion engines that are exposed to high stresses, such as piston rings, are usually made from cast iron materials or cast iron alloys. Piston rings and particularly compression rings in high-performance engines are exposed to increasing stresses, including peak compression pressure, combustion temperature, EGR and lubrication film reduction among others, and which have a critical effect on their functional properties, such as wear scuff resistance, microwelding and corrosion resistance.
Unfortunately, cast iron materials according to the prior art are highly susceptible to breakage, and rings often break when the existing materials are used. Higher mechanical-dynamic loads result in shorter operating lives for piston rings. Running surfaces and flanks are subject to heavy wear for the same reasons.
Higher ignition pressures, reduced emissions and direct fuel injection contribute to increased loads on the piston rings. As a result, the piston material is damaged and deposits accumulate on it, particularly on the lower piston ring flank.
Having to deal with higher mechanical and dynamic loads on piston rings, more and more engine manufacturers are requesting piston rings that are made from high-grade steel (annealed and high-alloyed, such as the material 1.4112). In this context, iron materials containing less than 2.08% by weight carbon are classified as steel. If the carbon content is higher, the material is considered to be cast iron. Steel materials have better strength properties and ductile values than cast iron because their microstructures are not disrupted by free graphite.
The steels used most frequently to produce steel piston rings are high chrome alloy, martensitic steels. Steel piston rings are manufactured from profile wire. The profile wire is roundwound, cut to length and drawn over an “out-of-round” mandrel. On this mandrel, the piston ring is given its desired out-of-round shape in an annealing process, which also sets up the requisite tangential forces. A further disadvantage of manufacturing piston rings from steel is that above a certain diameter, it is no longer possible to produce (wind) rings from steel wire. In contrast, cast iron piston rings are already cast out of round, so they are ideally shaped from the outset.
Cast iron has a considerably lower melting temperature than steel. The difference may be as much as 350° C. depending on chemical composition. Cast iron is therefore easier to melt and cast, since a lower melting temperature means a lower casting temperature and thus also less shrinkage due to cooling, so that the case material has few blowholes and/or hot or cold cracks. A lower casting temperature also generates less stress on the moulding material (erosion, gas porosities, sand inclusions) and the furnace as well as lower melting costs.
The melting temperature of the iron material depends not only on its carbon content but also on the “degree of saturation”. The following formula, shown in simplified form, applies:Sc=C/(4.26−l/3(Si+P)).
The closer the degree of saturation is to 1, the lower the melting temperature is. In the case of cast iron, a degree of saturation of 1.0 is usually aimed for, wherein the cast iron has a melting temperature of 1150° C. The degree of saturation of steel is about 0.18 depending on its chemical composition. Eutectic steel has a melting temperature of 1500° C.
The degree of saturation can be influenced considerably by the Si or P content. For example a 3% by weight increase in silicon content has a similar effect to a 1% by weight increase in C content. It is thus possible to produce a steel material having a C content of 1% by weight and 9.78% by weight silicon that has the same melting temperature as cast iron with a degree of saturation of 1.0 (C: 3.26% by weight, Si: 3.0% by weight).
If the Si content is increased significantly, the degree of saturation of the steel material may also be increased and the melting temperature lowered to the same level as cast iron. In this way, it is possible to produce steel using the same equipment as is used to produce cast iron, for example GOE 44.
Piston rings made from a steel casting material with high silicon content are known in the prior art. However, the presence of a larger quantity of silicon has a negative effect on the hardenability of the material, because its “Ac3” austenitic conversion temperature is raised.